Purpose: The aim of this study was to estimate normal tissue complication probability (NTCP) parametersfor late rectal complications after carbon ion radiotherapy (C-ion RT) for prostate cancer.Methods and materials: A total of 163 patients were used to derive NTCP parameters. These patients weretreated with relative biological effectiveness (RBE)-weighted dose ranging from 57.6 Gy (RBE) up to 72 Gy(RBE) and included in dose escalation trials. The Lyman–Kutcher–Burman (LKB) model was used and themodel parameters were fit to the relation between dose and complication observed after C-ion RT.Results: The resulting NTCP parameters were the volume effect parameter; n = 0.035 (95% CI: 0.024–0.047), the steepness of the NTCP curve; m = 0.10 (0.084–0.13), the tolerance dose associated with 50%probability of complication; TD50 = 63.6 Gy (RBE) (61.8–65.4 Gy (RBE)) for GradeP1, n = 0.012(0.0050–0.023), m = 0.046 (0.033–0.062), TD50 = 69.1 Gy (RBE) (67.6–70.9 Gy (RBE)) for GradeP2.Conclusion: A new set of rectal NTCP parameters in C-ion RT was determined. The rather small n valuessuggest that the rectum was consistent with being strictly serial organ. The new derived parameter valuesfacilitate estimation of rectal NTCP in C-ion RT.